Many methods for taking three-dimensional optical measurements of free-form surfaces provide for implementing a structured illumination of the test object. In this context, the test objects can have cylindrical or spherical and thus very complex and angled surfaces, as are prevalent, in particular, in turbine blades or BLISKs. BLISKs denote what are commonly referred to as bladed disks, as used in the compressor stages in jet engines, for example. The three-dimensional optical measurement of the surfaces may be used, on the one hand, to control dimensional accuracies, as well as to generate data records for creating electronic volume models of existing test objects. The structured illumination of the test objects encompasses fringe structures, dots or stochastic patterns, the light backscattered by the surface being recorded by a camera.
The German Patent DE 195 36 294 C2 describes applying a geometric navigation method for use with optical 3D sensors in order to take three-dimensional measurements of objects. This method is based on the principles of fringe projection and triangulation, using at least one camera, preferably a video camera, a device for digitizing and storing image sequences of the camera, at least one illumination projector, which is fixed in position relative to the camera, respectively is assigned to a plurality of cameras and which, in temporal succession, generates light structures composed of at least one-dimensional fringes, and using at least one navigation device that is fixed in the reference coordinate system and includes either reflecting and/or scattering signal markings, which are indicated by identification indices, along with an illumination device or a signal continuum. A method of this kind, which provides for using light structures to illuminate the test object surface to be measured, presupposes a surface that does not have any or that only has minimal specular reflectance. A specular property connotes, in particular, a high reflection, as is characteristic of metallic surfaces. However, a reliable implementation of the mentioned method requires that the surface have a mat, diffuse scattering reflective property. For that reason, the surface of the test object to be measured is covered with a coating that alters the optical properties of the surface in the manner mentioned above. Generally, spray (air brush) techniques are used to apply the coating, the applied material forming a white, diffusely scattering layer on the surface of the test object. This layer may include titanium dioxide, for example, since this material produces the desired properties.
The inherent difficulty that the known method must seek to overcome is ensuring that the measurement result obtained by optically measuring the free-form surfaces is not adversely affected by the coating on the surface. It is, therefore, essential that the layer be applied thinly and uniformly to the surface and, moreover, that it be readily removable again therefrom. A variation in the layer thickness results in a falsification of the measurement result, it not being possible for the coating material to be uniformly applied using the spray technique insofar as, when working with complex surface structures, such as turbine blades, for example, surfaces disposed in the inner regions or in recesses, are wetted to a lesser extent by the coating material. On the other hand, outer surfaces, such as the upper ends of turbine blades, for example, have a greater layer thickness, since, in these regions, the coating material is deposited more heavily on the surface. A single, integrated and uniform coating of the entire surface using a spray technique in accordance with the related art also necessitates producing a layer thickness that exceeds a value of 0.01 mm at least in some areas. In this instance, due to the large layer thickness, a precise measurement of the test object is no longer feasible, thereby resulting in measuring errors and thus in the generation of erroneous data records.